1. Field of the Invention
The present invention relates to a coil spring modeling apparatus capable of producing a reactive force (repulsive force) corresponding to compression of a helical spring such as a suspension coil spring.
2. Description of the Related Art
As an example of a vehicle suspension system, a McPherson-strut-type suspension is known. The McPherson-strut-type suspension comprises a coil spring, and a strut (a shock absorber) provided inside of the coil spring. The coil spring is compressed by a load applied from above the coil spring, and is extended and retracted in accordance with the load. The strut is also extended and retracted.
In the McPherson-strut-type suspension, in order to reduce the sliding resistance of a strut, offsetting a force line position (FLP) of a coil spring from the center line of the coil spring is known. For example, the force line position (FLP) of a coil spring is set at a position where the friction of the strut is minimal. For this reason, the relationship between a force line position (FLP) of a coil spring and the sliding resistance of a strut must be specified. However, producing a variety of coil springs whose force line positions are different by way of trial is time consuming and costly. Thus, instead of using the coil spring, using a coil spring modeling apparatus has been proposed.
For example, a coil spring modeling apparatus disclosed in, U.S. Pat. No. 7,606,690 (Document 1) is known. Also, an improved coil spring modeling apparatus is disclosed in “Research of Effect of Coil Spring Reaction Force Line on Vehicle Characteristics by Universal Spring” (Document 2), on pages 21 to 24 of the proceedings, presentation of which was made in the conference held by the Japan Society of Spring Engineers (in Nagoya) on Nov. 1, 2013, and “Experimental Study on the Effect of Coil Spring Reaction Force Vector on Suspension Characteristics” of SAE 2014 (Document 3), presentation of which was made in the U.S. (Detroit) on Apr. 8, 2014. The coil spring modeling apparatus disclosed in the above documents has a Stewart-platform-type parallel mechanism comprising six hydraulic cylinders. By actuating each of the hydraulic cylinders by fluid pressure, a reactive force corresponding to compression of a coil spring can be produced.
The coil spring modeling apparatus of Document 1 detects a reactive force which each of the hydraulic cylinders is subjected to by six load cells provided in the six hydraulic cylinders, respectively. The coil spring modeling apparatus disclosed in Documents 2 and 3 comprises external multi-axial load cells. By the external multi-axial load cells, friction of the strut, that is, the damper friction, is detected.
In the McPherson-strut-type suspension, when a coil spring is compressed between the lower spring seat and the upper spring seat, it is known that a relative change of rotational position is produced between the lower end turn portion and the upper end turn portion in accordance with the amount of compression. If the relative change of rotational position is restrained by friction, a moment around a kingpin axis (a kingpin moment) is produced in the suspension. The kingpin moment (KPM) becomes a factor which adversely affects the steering performance of a vehicle.
The kingpin moment (KPM) changes in accordance with a geometric positional relationship between the kingpin axis and the strut axis. Also, the kingpin moment (KPM) may sometimes be affected by a force line position (FLP). Accordingly, attempts have been made to mimic the force line position (FLP) of various coil springs and the kingpin moment (KPM) by the coil spring modeling apparatus. However, if the upper end of an actuator unit comprising a Stewart-platform-type parallel mechanism is restricted to a base member, when a torque about the axis is produced by the actuator unit, the actuator unit itself is twisted. Thus, what was conceived in the past was to produce a moment corresponding to a kingpin moment (KPM) by controlling a torque produced by the actuator unit. However, the conventional coil spring modeling apparatus leaves room for improvement in that detecting a kingpin moment (KPM) corresponding to that of the actual suspension with high accuracy is difficult.